Amortized Cost and Fair Value by Maturity Periods for Fixed Maturities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 413.0
Due after one year through five years, Amortized Cost	2,319.4
Due after five years through ten years, Amortized Cost	1,750.9
Due after ten years, Amortized Cost	705.4
Total fixed maturities, Amortized Cost	5,188.7
Mortgage-backed and asset-backed securities, Amortized Cost	1,340.8
Total fixed maturities, Amortized Cost	6,529.5	6,008.7
Due in one year or less, Fair Value	419.5
Due after one year through five years, Fair Value	2,442.8
Due after five years through ten years, Fair Value	1,919.3
Due after ten years, Fair Value	766.6
Total fixed maturities, Fair Value	5,548.2
Mortgage-backed and asset-backed securities, Fair Value	1,404.0
Fixed maturities, Fair Value	$ 6,952.2	$ 6,284.7